This invention relates to the sampling and determining of chemical substances by a self calibrating system. More particularly, this invention relates to a method of sampling and determining the concentration of diffusible chemical substances in a matrix wherein said determinations are made on a substantially continuous basis.
The determination of chemical substances such as blood gases by an in vivo monitoring system has been proposed by several different methods each of which has its inherent disadvantages.
An intermittent monitoring system for blood gases and pH is disclosed by Clark et al in U.S. Pat. Nos. 3,838,682 and 3,910,256. These patents disclose a system wherein blood is automatically and intermittently withdrawn from an arterial catheter and delivered to a fully automated blood gas analyzer. The system provides an reliable around the clock gas analysis system proven to be reliable for blood gas measurements. The main disadvantage of this system is the practical limitation on sampling frequency in that the system, especially in children, is limited by the blood loss and replacement problems as well as the amount of saline infusion required to maintain the patency of the catheter. A needed addition to the automated blood gas analysis concept is a practical continuous monitoring of the partial pressures of oxygen and carbon dioxide as well as other chemicals in the blood or other body fluids such as electrolytes and simple sugars. In addition, a system that does not require the withdrawal of blood would also be desirable.
Several in vivo methods of determining the partial pressures of blood gases have been taught in the prior art. Gardner et al, The Journal of Thoracic and Cardiovascular Surgery, Vol. 62, No. 6, pages 844-850 (1971) refer to a method of sampling blood gases in vivo using a Teflon-encased cannula. The coating material acts as a diffusion membrane and a mass spectrometer sampling apparatus containing a high-vacuum pump draws gas into the mass spectrometer from the tissues surrounding the cannula diffusing through the membrane coating in amounts which are purportedly proportional to the partial pressure in the tissue of the substance being monitored. The cost of a mass spectrometer and the size of the sample required are prohibitive for other than research use.
Brantigan U.S. Pat. No. 4,016,863 teaches an apparatus for the in vivo sampling of blood gases which allows the blood gases to diffuse through a gas permeable membrane and to come into equilibrium with a liquid contained within the membrane. The sampling is done on a "one shot" basis as the apparatus must be withdrawn from the subject and the equilibrated fluid analyzed elsewhere.
Myers et al, Surgery, Vol. 71, No. 1, pp 15-21, (1972), and Niinikoski et al, Surgery, Vol. 71, No. 1, pp 22-26, (1972), both teach methods of utilizing an implanted silicone polymer tube filled with saline as a diffusion fluid and allowing tissue gases to equilibrate through the tubing with the saline solution. The saline is left in the tubing sufficiently long to allow equilibrium to take place whereupon samples are withdrawn from the opposite end of the tubing for analysis. The requirements for making an incision and implanting the tubing along with the size and length of tubing implanted make these methods impractical.
Another in vivo method and apparatus for blood gas analysis is taught by Sielaff et al in U.S. Pat. No. 3,983,864 wherein a catheter containing a gas permeable membrane is inserted into an artery. A carrier gas such as helium is contained within the catheter and purportedly allows the blood gases to diffuse through the membrane and come into equilibrium with the carrier gas. By means of displacement the carrier gas containing the equilibrated gas is removed to another area for analysis. Distinct disadvantages are that the sampling is intermittent and the carrier gas also diffuses out of the membrane and into the blood.